The present invention relates to an art of amplifying a stereo sound image by two speakers through inputting stereo signal of two channels, an L signal (left signal) and R signal (right signal). More particularly, the present invention relates to a method and apparatus for processing a sound signal whose sound image is normally localized outside of two speakers through especially enhancing a difference signal between the L signal and R signal and adding a crosstalk cancellation process of short delay time.
Audio frequencies in which a person can hear are concentrated around 300 Hz to 3.5 kHz, and an audio frequency which is important for clear conversations is 1 kHz and its wavelength is 30 cm. Accordingly, for example, when a sound arrives from a left side direction of a head, the sound arrives as an inverse phase to a right ear from a left ear, since the right ear is positioned approximately 15 cm apart from left ear. Therefore, when a same sound arrives from left and right sides, a listener feels that the sound image is located in front of the listener.
However, since an L−R signal generates phase modulation and a volume difference against the R signal, a sound image of the L−R signal around 1 kHz is localized to the left side within a range of left 180 degrees, and a person feels the sound image arrives from direct left side. Also, since an R−L signal generates phase modulation and a volume difference against the L signal, a sound image of the R−L signal around 1 kHz is localized to the right side within a range of right 180 degrees, and a person feels the sound image arrives from direct right side.
FIG. 2 shows a conventional surround reproduction circuit (for example, see Japanese Unexamined Patent Publication No. 354595/2002). An L side surround signal, which is the L−R signal, is generated by an adder 35 through the L signal and R signal of a stereo being inputted from input terminals 31 and 32. The L−R signal, which is mainly a composition of high frequency of small vocal and echo sound, is inputted into a low-pass filter 36 and removes a harsh signal component, and then volume to be added to an original signal is adjusted by an operational amplifier 37. Then, the surround effect is obtained through adding the L−R signal component to the original L signal as an L side surround signal through an adder 38 and adding the R−L component signal to the original R signal as an R side surround signal through an adder 39, then the surround signal is outputted through output terminals 42 and 43. Numerals 33, 34, 40 and 41 are buffers.
As mentioned above, surround effect has been obtained through enhanced phase modulation and volume difference by removing sound signal component, which is enhanced by an ear and whose sense of direction is easily recognized by a person. Then, reverberation sound and reflected sound, which locate in a frequency band where the sense of direction is hard to be recognized, are enhanced and mixed with the L signal and R signal.
Since the surround reproduction circuit of a conventional art has been enhanced the L−R difference signal component, a sound signal, which is received in an ear, is not only a signal from one speaker but it is a result of the sum of signals from two speakers. Accordingly, there has been a problem that even if there is a sense of spread, the localized position becomes unclear since a phase frequency characteristic is disturbed at the ear.